1. Field of the Invention
This invention relates generally to field effect transistors (FETs), and, more particularly, to junction field effect transistors (JFETs).
2. Description of the Related Art
Junction field effect transistors (JFETs) are found in many applications, such as electronic switches and voltage-controlled resistances. The structure of a conventional JFET is shown in FIG. 1. This P-type JFET comprises an N-type bottom gate layer 10, P+ source and drain regions 14 and 16 recessed into N-type layer 10, a P-type layer 18 recessed into the surface of the N layer between the source and drain regions, and an N-type top gate layer 20 recessed into the surface of P-type layer 18 between the source and drain regions. Contacts are provided to the source and drain regions and the gate layers to provide the device's source (S), drain (D) and gate (G) terminals. Layer 18 serves as a P-channel which provides a current path between the source and drain regions.
With no voltage applied to gate terminal G, current flows easily when a voltage is applied between the source and drain terminals. Current flow is modulated by applying a voltage (Vgs) between gate terminal G and source terminal S. The polarity of Vgs is such that it reverse-biases the p-n junction between the gate and channel. This creates a depletion region which extends into the channel. The width of the depletion region varies with Vgs, with an increasing reverse-bias serving to widen the depletion region and thereby pinch off the channel and reduce the device's drain current. In this way, the Vgs voltage controls the conductance of the channel.
One inherent disadvantage of the JFET is its non-zero gate current. Voltages applied to the JFET's drain terminal give rise to an electric field (e-field) at its drain/P-channel junction. The e-field causes impact ionization to occur at the junction, which leads to the generation of carriers that are swept into the gate—thereby creating a gate current which may increase to an unacceptably high level with increasing drain voltage.
The e-field at the drain/channel (or source/channel) junction can also result in the device having a poor breakdown voltage characteristic. When the e-field reaches a critical level, the impact ionization current is essentially so high that drain current increases almost independently of drain voltage; this critical level defines the device's breakdown voltage, which may be unacceptably low for some JFETs.